Ummins ISX15 Random Overheating

[schISM]

You didn't state in your post whether or not you've had to add any coolant before or after the issue arose. It sounds like your running in a warm region if your running the A/C so maybe you didn't check to see if the heat was blowing hot when it was overheating. If it doesn't I'd have the system pressurized again and watch the tech. do it and see the results first hand.